1. Field of the Invention
The present invention relates to a variable capacitor, and particularly to a variable capacitor in which the effective overlapping area between a stator electrode and a rotor electrode is varied through rotation of the rotor electrode relative to the stator electrode to thereby vary capacitance.
2. Description of the Related Art
One type of variable capacitor is disclosed in Japanese Patent Application Laid-Open (kokai) No. 6-120079. FIGS. 13(A) and 13(B) show the variable capacitor disclosed in that publication.
As shown in FIGS. 13(A) and 13(B), a variable capacitor 1 is composed of a stator 3 and a rotor 5. The stator 3 has a stator electrode 2 formed therein and is formed of a ceramic dielectric. The rotor 5 has a rotor electrode 4 formed on its bottom side (as viewed in FIG. 13(B)) and is formed of a metal. The rotor electrode 4 faces the stator electrode 2 via a portion of the ceramic dielectric constituting the stator 3.
A metallic cover 6 secures the rotor 5 so that it can be rotated relative to the stator 3 so as to vary the effective overlapping area between the rotor electrode 4 and the stator electrode 2. The cover 6 is shaped so as to accommodate the rotor 5 rotatably, and is attached onto the stator 3.
The cover 6 has an adjustment hole 7 formed therein so as to permit insertion of a tool for rotating the rotor 5. The cover 6 also has a spring-action portion 8 formed around the adjustment hole 7 for pressing the rotor 5 against the stator 3. The spring-action portion 8 is shaped in such as manner as to incline downward (as viewed in FIG. 13(B)) toward the center of the adjustment hole 7, thereby applying a spring force by means of a metallic material present around the adjustment hole 7.
In the thus-configured variable capacitor 1, in order to form capacitance between the stator electrode 2 and the rotor electrode 4, which oppose each other, a stator terminal 9 and a rotor terminal 10 are provided. The stator terminal 9 is provided on an end surface of the stator 3 and is formed of a conductive film which is electrically connected to the stator electrode 2. The rotor terminal 10 is integrally formed with the conductive cover 6, which is in contact with the rotor 5, with the rotor electrode 4 formed thereon. Accordingly, the capacitance of this device can be sampled (e.g., connected to an external circuit) between the stator terminal 9 and the rotor terminal 10. Through rotation of the rotor 5, the effective overlapping area between the stator electrode 2 and the rotor electrode 4 is varied, so that the capacitance is varied accordingly.
Another variable capacitor is described in Japanese Patent Application No. 9-126586 filed on May 16, 1997 by the applicant of the present invention. FIG. 15 shows a variable capacitor 101 disclosed in that application for patent.
As shown in FIG. 15, the variable capacitor 101 is primarily composed of a stator 102, a rotor 103, and a cover 104. A major portion of the stator 102 is formed of a dielectric, such as ceramic. The rotor 103 is formed of a metal, such as brass. The cover 104 is formed of a metal, such as stainless steel or copper alloy.
The above-mentioned elements of the variable capacitor 101 will next be described in detail.
The stator 102 generally has a symmetrical structure. Stator electrodes 105 and 106 are formed side by side in the stator 102. Stator terminals 107 and 108 are formed of a conductive film on the outer surfaces of corresponding end portions of the stator 102 so as to establish electrical connection with the stator electrodes 105 and 106, respectively. A portion of the dielectric, which the stator 102 is made of, defines a dielectric 109 covering the stator electrodes 105 and 106.
As described above, the two stator electrodes 105 and 106 and the two stator terminals 107 and 108 are formed so as to impart a symmetrical structure to the stator 102, so that orientation of the stator 102 need not be a consideration during the assembly of the variable capacitor 101.
The rotor 103 is placed on the stator 102 in such a manner that the rotor 103 comes into contact with the outer surface of the dielectric layer 109. A substantially semicircular rotor electrode 111 projects from the bottom side (as viewed in FIG. 15) of the rotor 103 so as to face the stator electrode 105 (or 106) with the dielectric layer 109 disposed therebetween. FIG. 16 shows a bottom view of the rotor 103.
A protrustion 112 is also included, which projects out as far as the rotor electrode 111. This protrustion 112 is formed on the bottom side of the rotor 103 in a region other than that where the rotor electrode 111 is formed. The protrusion 112 serves to prevent an inclination of the rotor 103 which would otherwise result due to the presence of the rotor electrode 111.
A driver groove 113, which assumes a form of, for example, a square through-hole, is formed in the rotor 103 in order to receive a driver or a like tool used for rotating the rotor 103.
The cover 104 is attached onto the stator 102 while accommodating the rotor 103. The cover 104 holds the rotor 103 so that the rotor 103 can rotate relative to the stator 102. The cover 104 has an adjustment hole 114 formed therein that allows the driver groove 113 to be exposed therethrough. Thus, when the rotor 103 is to be rotated, a driver or a tool can be inserted into the driver groove 113 through the adjustment hole 114.
The cover 114 has a spring-action portion 115 formed around the adjustment hole 114. The spring-action portion 115 is partially in contact with the upper surface (as viewed in FIG. 15) of the rotor 103 to thereby apply a spring force for pressing the rotor 103 against the stator 102. The spring-action portion 115 is formed in such a manner as to incline downward toward the center of the adjustment hole 114, thereby affecting a spring force by means of a metallic material present around the adjustment hole 114.
A plurality of protrusions 116 are formed on the spring-action portion 115 at equal intervals along a rotational direction of the rotor 103 (e.g., a direction defined by the rotation of the rotor 103). These protrusions 116 substantially come into point contact with the rotor 103. These protrusions 116 can be formed through, for example, embossing a metallic plate which constitutes the cover 104.
The cover 104 also has a rotor terminal 117 extending downward (as viewed in FIG. 15).
Through use of the above-mentioned stator 102, rotor 103, and cover 104, the variable capacitor 101 is assembled in the following manner.
The rotor 103 is placed on the stator 102, and then the cover 104 is placed on the stator 102 in such a manner as to cover the rotor 103. Next, the cover 104 is attached onto the stator 102 while being pressed toward the stator 102 so as to press the rotor 103 against the stator 102.
In this case, the rotor terminal 117 integrated with the cover 104 is positioned to face the stator terminal 108 provided on the stator 102. In the structure illustrated in FIG. 15, the stator terminal 108 does not function as a stator terminal in the same manner as stator terminal 107, and thus no electrical problem will arise due the above-described electrical connection.
In the thus-assembled state, the rotor electrode 111 faces the stator electrode 105 with the dielectric layer 109 disposed therebetween to thereby develop capacitance. In order to vary the capacitance through varying the effective overlapping area between the rotor electrode 111 and the stator electrode 105, the rotor 103 is rotated. The capacitance can be externally "tapped" between the stator terminal 107 and the rotor terminal 117. The stator terminal 107 is electrically connected to the stator electrode 105. The rotor terminal 117 is integrated with the cover 104 which is in contact with the rotor 103 on which the rotor electrode 171 is formed.
In the variable capacitor 101, the protrusions 116 formed on the spring-action portion 115 of the cover 104 are substantially in point contact with the rotor 103. Accordingly, the protrusions 116 press the rotor 103 at reliably fixed positions. Even when the parallelism of the rotor 103 between the rotor-electrode side and the opposite side is poor or when the flatness of the rotor-electrode side or the opposite side of the rotor 103 or the flatness of a tip portion of the spring-action portion 115 is poor, a contact pressure can be applied in a stable manner to the rotor 103. In other words, these variations in machining do not appreciably affect the performance of the variable capacitance and are thereby effectively "absorbed."
Thus, the rotor 103 is uniformly pressed against the stator 102 over the entire surface of the rotor 103. Therefore, the capacitance of the variable capacitor 101 is stabilized and varies smoothly with rotation of the rotor 103. Also, the drift in the set position is stabilized and the torque required to rotate the rotor 103 becomes uniform.
However, the above-described variable capacitor 1 involves at least the following problems.
As mentioned previously, the cover 6 is shaped so as to accommodate the rotor 5, producing a resultant opposed arrangement between an edge portion 11 of the cover 6 and the stator 3.
As mentioned previously, a conductive film formed on an end surface of the stator 3 serves as the stator terminal 9. The conductive film is generally formed in the following manner. The corresponding end portion of the stator 3 is thrust into a conductive paste layer having a predetermined thickness to thereby apply the conductive paste onto the end portion of the stator 3. The thus-applied conductive paste is then subjected to baking so that it becomes fixed to the stator 3.
As a result of forming the stator terminal 9 by the above-mentioned method, as seen well in FIG. 14 showing a partially enlarged view of FIG. 13(B), the stator terminal 9 formed on an end surface of the stator 3 partially extends onto surfaces adjacent to the end surface. Particularly, with regard to a surface 12 of the stator 3 which faces the lower surface of the rotor 5, the stator terminal 9 includes a portion 13 extending onto the surface 12.
Accordingly, the edge portion 11 of the cover 6 or a circumferential surface of the rotor 5 directly faces the portion 13 of the stator terminal 9. In the variable capacitor 1, the cover 6 and the rotor 5 have an electric potential identical to that of the rotor electrode 4, while the stator terminal 9 has an electric potential identical to that of the stator electrode 2. As a result, if a short circuit occurs between the portion 13 of the stator terminal 9 and the edge portion 11 of the cover 6 or between the portion 13 of the stator terminal 9 and the circumferential surface of the rotor 5, the variable capacitor 1 may be disabled. In order to prevent such a short circuit or to improve dielectric strength, the distance between the portion 13 of the stator terminal 9 and the edge portion 11 of the cover 6 or that between the portion 13 of the stator terminal 9 and the circumferential surface of the rotor 15 is increased as much as possible.
However, when the size or height of the variable capacitor 1 is to be reduced, the distance between the portion 13 of the stator terminal 9 and the edge portion 11 of the cover 6 or that between the portion 13 and the circumferential surface of the rotor 5 may be unavoidably decreased. The size of the variable capacitor 1 can be effectively reduced through reduction of, for example, a dimension of the stator 3 as measured in a right-and-left direction in FIG. 13(A) or 13(B). In this case, if the diameter of the rotor 5 is reduced, an available maximum capacitance decreases; thus, a reduction in the diameter of the rotor 5 is avoided as much as possible in order to maintain a certain maximum capacitance. Accordingly, when the size of the variable capacitor 1 is to be reduced while a certain maximum capacitance is maintained, only the dimension of the stator 3 as measured in the right-and-left direction of FIG. 13(A) or 13(B) is reduced. As a result, the distance between the portion 13 of the stator terminal 9 and the edge portion 11 of the cover 6 and the distance between the portion 13 of the stator terminal 9 and the circumferential surface of the rotor 5 are unavoidably decreased.
Consequently, if there occurs a positioning error between the stator 3 and the rotor 5 or between the stator 3 and the cover 6 in assembly of the variable capacitor 1, or a dislocation of the cover 6 or rotor 5 during rotation of the rotor 5, a short circuit or deterioration of the dielectric strength occurs between the portion 13 of the stator terminal 9 and the end portion 11 of the cover 6 or between the portion 13 and the circumferential surface of the rotor 5.
Further, when the variable capacitor 101 is to be made thinner, thinning of the rotor 103 is effective. However, when the rotor 103 is thinned to a thickness of 0.3 mm or less, a pressing force applied to the rotor 103 by the spring-action portion 115 may cause the rotor 103 to deform. Particularly, as in the case of the variable capacitor 101 shown in FIG. 15 in which protrusions 116 are formed on the spring-action portion 115, the pressing force is applied to the rotor 103 in a localized narrow region. Thus, the rotor 103 is known to be susceptible to deformation. Such an undesirable deformation of the rotor 103 hinders smooth capacitance variation affected through rotation of the rotor 103, typically causing a problem that the linearity of the capacitance variation is impaired, causing, in turn, a resultant drift in setting position.
In FIG. 15, the dot-and-dash line represents the position of a section of the spring-action portion 115 of the cover 104, which section comes into contact with the rotor 103. More specifically, the line represents the position of a contact portion 118 of the protrusion 116 formed on the spring-action portion 115, which contact portion 118 comes into contact with the rotor 103. As the rotor 103 rotates, the contact portion 118 sweeps out a circular trajectory 119 on the rotor 103 as represented by the dot-and-dash line in FIG. 16. FIG. 16 is a bottom view of the rotor 103 of FIG. 15. The circular trajectory 119 is a projection on the bottom side of the rotor 103 of a circular trajectory which is swept out on the upper surface (as viewed in FIG. 15) of the rotor 103 by the contact portion 118.
As shown in FIGS. 15 and 16, the protrusion 112 formed on the bottom side of the rotor 103 is not positioned on the circular trajectory 119, but is positioned outside the circular trajectory 119. Accordingly, a pressing force applied to the rotor 103 by the spring-action portion 115 through the protrusion 116 acts on a portion of the rotor 103 located between the protrusion 112 and the rotor electrode 111, causing the rotor 103 to deform in a flexural manner.